1. Field of the Invention
The present invention relates to an apparatus for and method of stably controlling an optical interferometer of a receiver in a differential phase shift keying (DPSK) system, and more particularly, to an apparatus for and method of stably controlling a 1-bit delay Mach-Zehnder interferometer (MZI) by automatically and optimally setting a thermoelectric cooler (TEC) and a precision adjustor on one path such that the 1-bit delay MZI has an optimal path difference and by automatically following an optimal point such that the 1-bit delay MZI maintains optimal characteristics regardless of external change such as the change in input power or input wavelength.
2. Description of the Related Art
For optical transmission systems using wavelength division multiplexing (WDM), various types of modulation having better transmission characteristics than conventional nonreturn-to-zero (NRZ) modulation have been suggested. One of those is DPSK. As compared to conventional intensity modulation, the DPSK provides improved receiving sensitivity and is robust to the nonlinearity of optical fiber and is thus suitable for remote transmission.
In DPSK, only phase of an optical signal is modulated with the intensity of the optical signal maintained constant. Accordingly, to directly detect the optical signal using a photodetector in a receiver, it is needed to convert phase modulation into intensity modulation. A device performing this conversion is a 1-bit delay MZI. Since the 1-bit delay MZI has transmission characteristics depending on an input wavelength, a temperature control circuit is essential thereto to adjust and maintain the 1-bit delay of one path. The transmission characteristics of the 1-bit delay MZI vary with a TEC that controls entire module temperature and a precision adjuster that precisely adjusts the length of one path. Even if the 1-bit delay MZI is initially set to have optimal transmission characteristics, it cannot maintain optimal performance when an input wavelength changes due to external changes during operation. Accordingly, to commercialize the 1-bit delay MZI, a method of stably controlling the 1-bit delay MZI by automatically controlling a precise adjuster such that the 1-bit delay MZI initially has the optimal transmission characteristics with respect to channels of a WDM system and by continuously and automatically following values set for the optimal transmission characteristics during operation such that the transmission characteristics do not deteriorate due to external changes is essential.
Many studies have been underway in the field of DPSK but have not reached a commercialization stage yet. In particular, since development of techniques essential to commercialization is under progression, there are not many relevant patents or papers published. In a conventional method of stably controlling an interferometer, a transmitter leaves a part of a carrier component by reducing the magnitude of a modulation drive voltage so that a receiver detects output power of the carrier component and stabilizes a 1-bit delay MZI. However, this conventional method has a poor extinction ratio and depends on input power. Consequently, it is difficult to use the conventional method in commercialized systems.